Table 1.

Thymoma Classification Systems

Classification System
Lattes/Bernatz	Traditional description of the dominant cell type
Muller-Hermelink	Presumed origin of the malignant cell (corticomedullary classification)
WHO	Based on the traditional descriptive classification and the corticomedullary classification
	WHO type	Histologic criteria
	A	Bland spindle/oval epithelial tumor cells with few or no lymphocytes. (Synonyms: spindle cell thymoma; medullary thymoma)
	AB	Mixture of a lymphocyte-poor type A thymoma component and a more lymphocyte-rich type B-like component. (Synonyms: mixed thymoma)
	B1	Histological appearance of normal thymus, composed predominantly of areas resembling cortex with epithelial cells scattered in a predominant population of immature lymphocytes, and areas of medullary differentiation. (Synonyms: lymphocyte-rich thymoma; predominantly cortical thymoma)
	B2	Large, polygonal tumor cells that are arranged in a loose network and exhibit large vesicular nuclei with prominent large nucleoli – background population of immature T-cells always present. (Synonyms: cortical thymoma)
	B3	Predominantly medium-sized round or polygonal cells with slight atypia. Epithelial cells are mixed with minor component of intraepithelial lymphocytes. (Synonyms: well-differentiated thymic carcinoma; epithelial thymoma; squamoid thymoma)
	C	Heterogeneous group of thymic carcinomas
Masaoka	Presence of invasion and anatomic extent of involvement (clinically and histopathologically)
	I	Macroscopically completely encapsulated and microscopically no capsular invasion
	II	Microscopic invasion into capsule (IIa) or macroscopic invasion into surrounding fatty tissue or mediastinal pleura (IIb)
	III	Macroscopic invasion into neighboring organs (ie, pericardium, great vessels, or lung)
	IVa	Pleural or pericardial dissemination
	IVb	Lymphogenous or hematogenous metastasis

WHO = World Health Organization.